1. Field of the Invention
The invention relates to a retensioning device for strapping machines.
2. Background Art
It should be mentioned as the background of the invention that strapping machines normally have a tape guide frame which is constructed vertically on a work table and in which a stack of products, for example a stack of periodicals, to be strapped with a tape is introduced. A strapping tape is then pulled off a supply roll by a tape drive device and shot into the tape guide frame until the leading end of the tape enters a welding arrangement beneath the stack of products in the work table. The leading end is held and the strapping tape retracted by a retractor. It issues from the tape guide frame and wraps itself round the stack of products.
In this state, the strapping tape has to be tightened round the stack of products with a defined retensioning force. This retensioning force is controlled for various reasons and is difficult in many respects. It can basically be assumed that retensioning takes place with a pair of tape tensioning rollers through whose nip the strapping tape passes. The contact between strapping tape and pair of rollers accordingly has to be controlled in such a way that the force exerted on the tape and causing the retensioning can be defined. This is due to the technical requirement of strapping, for example, a stack of periodicals with a few copies only with a slight retensioning force owing to its low flexibility, whereas a high stack of periodicals is to be strapped with a far greater retensioning force owing to the compressibility of the periodicals and the much greater stability of the stack to kinking of the individual volumes. It must be possible to adjust the retensioning force individually from stack to stack as stacks which follow one another directly and have markedly differing heights have to be processed in the strapping machine in modern packaging lines of, for example, printing plant.
The retensioning force which can be produced in the strapping tape by the pair of tape tensioning rollers is accordingly dependent on very different parameters, for example the coefficient of friction between the strapping belt and the rollers, the force with which the two rollers are pressed toward one another and the angle of wrap with which the tape is generally guided round one of the two rollers. Furthermore, the torque provided by the rollers obviously has a significant influence on the retensioning force.
DE 32 49 559 C2 shows a feed and tensioning device for a strapping tape which is to be tensioned round a package. A revolving pressure roller is provided as counter roller which cooperates with a tensioning wheel, arranged on a pivoting lever, as pivot roller. Tape guide and angle of wrap of the retensioning device shown therein are designed in such a way that the pivoting lever is loaded by the retensioning force building up during the retensioning process via the pivoting roller in such a way that the pivoting roller is pressed against the counter roller. The feed force which can be exerted on the tape is therefore increased. The completion of the retensioning process and, in particular, the drive of the pivoting roller are controlled by the excursion of the pivoting lever. On attainment of a specific force, the pivoting lever is deflected so far from its spring-loaded basic position that a contact switch for the drive motor of the pivoting roller is actuated and the motor is switched off. The maximum retensioning force can be adjusted by an energy store in the form of a helical compression spring which loads the pivoting lever against its excursion direction. The bias of the helical compression spring and therefore the loading of the pivoting lever can be adjusted by means of a hand wheel. Therefore, the maximum retensioning force cannot be adjusted in an economical and automatable manner.
It can be seen that the above-described retensioning device according to the prior art has a very complex construction and can only be controlled with difficulty with respect to the adjustment and control of the retensioning process. This is all the more problematic since modem strapping machines operate with extremely short cycle times of less than one second. This means that the actual retensioning process lasts only one to two tenths of a second and therefore does not allow a time frame for complicated control measures.
Starting from the aforementioned problems, the object of the invention is to provide a retensioning device which, despite its simplicity of construction, makes possible proper control of the retensioning force, in particular also with respect to extremely quick retensioning processes.
This object is achieved by the loading device for the defined loading of the lever being accordingly coupled to the pivoting lever so that the pivoting roller can be adjusted toward the counter roller with a defined loading force. This loading force opposes the retensioning force building up in the tape during the retensioning process in such a way that the pivoting roller momentarily lifts from the counter roller when the loading force is exceeded and therefore interrupts the retensioning process. Owing to the lifting of the pivoting roller from the counter roller, the strapping tape becomes virtually free so the retensioning force collapses and the pivoting roller can be pressed back against the counter roller by means of the loading device. The tape is therefore retensioned again and the foregoing sequence of operation is repeated within the retensioning device. An equilibrium of forces is adjusted overall owing to the opposing forces on the lever, and the retensioning force which can be produced in the strapping tape can be controlled as a function of the loading force of the pivoting lever and by means of it. On attainment of the maximum retensioning force, a mechanical control circuit comes into effect in practice owing to the excursion of the pivoting lever and keeps the retensioning force almost constant during rotation of the roller.
Further features, details and advantages of the invention can be inferred from the following description in which an embodiment of the subject of the invention is described in more detail with reference to the accompanying drawings.